This invention relates to the scanning of photographic images, and more particularly in a primary application, to scanning in infrared and visible light in order to prepare for correction of surface defects.
FIG. 1 shows a prior art trilinear film scanner, and also introduces some terms that will be used in this application. A lamp 102 transilluminates a filmstrip 104 containing an image 106 to be scanned. Normally the light from the lamp 102 would be diffused or directed by additional optics, not shown, positioned between the lamp 102 and film 104 in order to illuminate the image 106 more uniformly. The image 106 on the film 104 is focused by lens 108 onto a sensor line 110 in a circuit package 112. The sensor line 110 projects back through lens 108 as a line 116 across the image 106. This line 116 is composed of many individual points, or pixels. To scan the entire image 106, the film 104 is moved perpendicularly to the line 116 to scan a two dimensional area, such as image 106. Because the sensors of the sensor line 110 are positioned in lines, this arrangement is called a linear, or line, sensor.
The sensor line 110 may be of a form known in the art as a xe2x80x9ctrilinearxe2x80x9d, or three line, array. As shown magnified at 120, the sensor line 110 actually consists of three parallel lines of sensors. In this prior art embodiment, one line of sensors 122 is behind a line of red filters 124. This arrangement could consist of a series of independent filters, but is normally a single long red filter 124 which covers all of the sensors of line 122. Another line of sensors 126 is behind a green filter line 128, and a third line of sensors 130 is behind a blue filter line 132.
As the film 104 is moved, the three lines 122, 126, and 130 each provide an individual image of the film seen with a different color of light. The data from the circuit package 112 is sent along cable 136 to supporting electronics and computer storage and processing means, shown together as computer 138. Inside computer 138 the data for each color image is grouped together, and the three images are registered as the three color planes 140, 142, and 144 of a full color image. Each of these color planes 140, 142 and 144 consists of pixels describing with a number the intensity of the light at each point in the film. For example, pixel 150 of the red color plane 144 may contain the number xe2x80x9c226xe2x80x9d to indicate a near white light intensity at point 152 on the film 104, as measured at a specific sensor 154 in the array 110, shown enlarged in circle 120 as sensor 156 behind the red filter line.
In FIG. 1 it is noted that there is a spacing between sensor lines 122, 126 and 130, and therefore the same point on the film 104 is not sensed by all three color lines at the same point in time. FIG. 2 illustrates this registration problem in more detail.
In FIG. 2 there is a trilinear array (not shown) with red, green, and blue sensor lines 202, 204, and 206. These lines are projected onto a substrate (not shown) which is moved in the direction of the arrow to scan out regions of the image on the substrate. The region seen by each line is different from the region seen by the other lines. For example, at the beginning of an arbitrary time interval, sensor 210 of the blue line 206 may see point 212 of the substrate, while at the end of the time interval, it may see point 214. It is apparent that each of the different sensors 210, 220 and 230 sees a different area during the same time interval. For example, at the end of the time interval, sensor 220 of the red sensor array 202 sees point 222, which is different than point 214 seen by the blue sensor 210 at the same end time. However, if the time interval is long enough, there will exist a region of overlap 224 over which all array lines have passed. If the interval between measurements is an integer submultiple of the spacing between the arrays, then there exists a time at which sensor 230 of the green line 204 sees the same point 232 on the substrate as 214, and another time at which sensor 220 of the red line 202 sees point 234, the same as point 214, which in turn will be seen by the blue sensor 210 at a later time. The computer system 138 receiving the information from the scans made by the trilinear array registers the data representing the three color images by shifting the data an amount corresponding to the distance between sensor lines, and discarding the part of each color record outside the full color range overlap 224.
Although this illustration has presented a so-called transmission, or film, scanner, a reflection, or print, scanner uses the same principles except that the source light is reflected from the same side as the imaging lens. As is explained later, there are uses for the present invention in both transmission and reflection scanners.
The conventional scanners described above scan in the three visible colors, exclusive of the invisible infrared. There are several reasons that it would be useful to add an infrared record registered to the conventional colored records. For example, examination of old documents under infrared with a reflection scanner is proving useful in examination of historic works, such as the Dead Sea Scrolls, to disclose alterations. Another potential use presented here without admission that it is known in the art, is to distinguish the xe2x80x9cKxe2x80x9d or black channel from the cyan, magenta, and yellow channels in a four color print. Currently a major commercial use of infrared plus visible scans is a technology called infrared surface defect correction, as explained in FIG. 3. Current applications of infrared surface defect correction are limited to transmission scanners, although it may be extended to reflection scanners, and therefore the specific illustration of a transmission scanner given below is not to be considered a limitation.
In FIG. 3, a lamp 302 transilluminates filmstrip 304 containing an image 306. An electronic camera 308 views the image 306 and outputs red, green, and blue digitized records 310, 312, and 314. In addition the electronic camera 308 outputs an infrared record 316. There are several ways a conventional camera can be made responsive to selectively visible and infrared light. One way is to provide a filter wheel 320 with four filters: red 322, green 324, blue 326, and infrared 328. If the camera 308 is a monochrome camera whose sensitivity extends into infrared, then the three visible colors and infrared may be captured at four different times, each time illuminating the film with a different filter in the filter wheel 320.
The cyan, magenta, and yellow dyes that create the image 306 are all transparent to infrared light, and therefore the film 304 appears clear to camera 308 when viewed under infrared light. On the other hand, surface defects such as dust, scratches, and fingerprints refract the light passing through the film 304 away from the camera 308, and therefore appear as darkened points under both visible and infrared light. Because refraction under infrared light is nearly equal to refraction under visible light, the defects appear nearly as dark in the infrared as in the visible spectrum.
Therefore infrared record 316 is effectively of a clear piece of film including defects, and image 310 contains the same defects plus the red image. The infrared image 316 provides a pixel by pixel xe2x80x9cnormingxe2x80x9d for the effect of defects. For example, defect-free pixel 340 in the red record 310 may contain a 50% brightness measurement. The corresponding defect-free pixel 342 in the infrared record 316 contains 100% brightness because no defect has attenuated the light. Function block 344 divides the 50% brightness level from the red record 310 by the norming 100% brightness level from the infrared record 316 to give a 50% brightness measurement for corrected pixel 346. On the other hand, pixel 350 under scratch 352 in the red record 310 may contain a 40% brightness measurement. The corresponding pixel 354 in the infrared record 316 seeing the same scratch may contain 80% brightness because the scratch has refracted 20% of the light. When function block 344 divides 40% by 80%, a corrected brightness value of 50% is determined for pixel 356. Note that corrected pixels 346 and 356 within the same background area of the image now both contain the same brightness value of 50%, so the effect of the scratch has disappeared. This division is repeated for each pixel to produce the corrected red record 360; and the same division by infrared is applied to the green record 312, and blue record 314, to produce the corrected green and blue records 362 and 364, resulting in a full color corrected image.
There are several ways of generating an infrared scan in conjunction with a visible scan. One method makes four passes across the original image using a light that changes color between passes, as was shown in FIG. 3. Unfortunately, this can take four times as long as a single pass scanner. Alternately, one can make a single pass while flashing four lights in rapid succession, but again the hardware may need to move at one fourth the speed. None of these prior art methods combines the speed obtained with a single pass multilinear array with the image clarity possible in the prior art attained by making multiple scans. It is apparent that the introduction of such a system would provide an improvement to the state of the art in infrared surface defect correction, as well as to the other uses of combined infrared and visible scans mentioned above.
The present invention adds a line to a conventional multilinear sensor array. The added line is specific to the infrared scan. In the most direct embodiment, the added line makes what was a trilinear array containing three lines, one for each of three primary colors, into a quadrilinear array. In a second embodiment, the red and blue sensor lines are combined into one line that alternates between red and blue sensors. This second embodiment uses only two lines for sensing full color, allowing the third line of existing trilinear layouts to be devoted to infrared.